The invention relates in general to intrusion alarm systems, and more particularly to portable, ultrasonic, pulse-echo type systems that generate an alarm signal upon the occurrence of an intrusion within a protected area.
Intrusion alarms of numerous types have been utilized for detecting the presence of an intruder within a protected area. In one known intrusion alarm system operating on the Doppler principle, a transmitter and receiver are located in or adjacent to an area to be protected. A continuous wave of ultrasonic energy is broadcast by the transmitter at a predetermined frequency, reflected by stationary objects in the protected area and then sensed by the receiver. Insofar as the reflected energy occurs at the same frequency, the apparatus determines that an intruding condition does not exist and no alarm is sounded. The passage or movement of an intruder through the protected area, however, causes a variation in the frequency of the reflected energy which variation is sensed and an alarm is sounded. This form of intrusion alarm system is recognized to be substantially susceptible to false alarms.
In another intrusion alarm system, pulse-echo techniques are employed wherein a pulsed signal of acoustical energy is transmitted by a transducer and the occurrence of a pulse of reflected acoustical energy within a predetermined time interval is indicative of the presence of an object in an area being examined. By transmitting pulses of acoustical energy in a narrow beam toward a reference surface and detecting reflections of the pulsed acoustical energy, any activities or environmental changes which alter a reflection from the reference surface are detected and an alarm is sounded. The pulse-echo intrusion alarm systems have been found to be of improved reliability and less susceptible to false alarms than systems operating on the Doppler principle. In addition, they lend themselves to a relatively low cost, mass production fabrication. Two such systems are disclosed in allowed copending application for U.S. Letters Patent entitled, "Improved Detection Method and Apparatus", inventor R. Salem, Ser. No. 959,105 filed Nov. 9, 1978 and U.S. Pat. No. 4,229,811, issued to R. Salem for "Improved Detection Method and Apparatus" on Oct. 21, 1980, both assigned to the present assignee.